This invention relates to a process of manufacturing vitreous beads in which particles of a vitrifiable material are vitrified and spherulized. The invention also relates to vitrifiable particles suitable for use in such a process.
Vitreous beads fall broadly into two main categories, namely solid beads and hollow beads. Beads in both categories find wide use as fillers for plastics materials for various purposes. Hollow beads are also used as fillers for certain explosives, especially those based on an aqueous emulsion, in order to increase their brisance, and deuterium filled hollow beads also find use as laser fusion targets. Solid beads are also incorporated in paint for the manufacture of reflective signs, for example road signs, and for shot- or sand-blasting.
It is well known to manufacture solid glass beads by spherulizing crushed glass cullet, for example originating from waste offcuts from a flat glass manufacturing plant.
In contrast to that simple process which is useful for forming solid glass beads, hollow beads are usually formed from a feedstock comprising pellets of a glass-former composition based on sodium silicate which may have been reacted with some other ingredient such as boric acid. These particles are vitrified and spherulized in a spherulizing furnace. The glass-former composition contains a substance which gives rise to the evolution of gas in such furnace with consequent cellulating effect.
The production of hollow vitreous beads for use as laser fusion targets, starting from a fragmented silica or silica-based gel, is referred to in "Processing of Gel Glasses" by Jerzy Zarzycki in "Glass Science and Technology" (Ed. D. P. Uhlmann & N. J. Kreidl, The Academic Press Inc. 1984) Volume 2, pages 213 to 245. The gel contains occluded water which evaporates when the particles are heated to bring about their vitrification and spherulization, so generating vapour pressure which has an expanding and cellulating effect on the glass beads.
The aforesaid prior processes for producing glass beads by vitrifying and spherulizing particles of a glass-former composition result in hollow vitreous beads having a rather high sodium ion content. As is well known in the glass manufacturing art, sodium compounds act as melting fluxes and promote low melting and vitrification temperatures so facilitating the manufacture of the glass. However the high alkali metal ion content renders the beads liable to attack by hydrolysis. The result of this is that initial adherence between the beads and any plastics materials in which they are used as filler is rapidly weakened and the ageing properties of a plastics or an explosive in which the beads are used as filler are consequently poor. In the case of a filled explosive the advantage of increased brisance is soon lost. The hydrolysis can cause perforation of the bead walls so rendering them useless as a filler for an explosive or as a laser fusion target.
In order to promote good ageing properties of various products incorporating hollow vitreous beads, it is known to subject the beads to an acid leaching treatment to reduce their alkali metal ion content, but such a dealkalising treatment adds to the cost and inconvenience of manufacture.
There is need for a more economic process by which vitreous beads with good resistance to hydrolysis can be produced. There is also a demand for glass beads having special properties not associated with the known high alkali beads, for example a high mechanical strength/bulk density ratio to serve as a low density filler capable of resisting the pressures involved in injection moulding or extruding filled plastics materials, or a high refractive index as required for beads to be used in light reflecting devices.
Vitrifiable compositions used in manufacturing low alkali glasses, because they are low in melting fluxes, require relatively high furnace temperatures for melting and vitrification. This is why such compositions have not been used as starting material for the production of hollow glass beads in a vitrifying and spherulizing furnace. Prima facie, an increased furnace temperature should be avoided because of the necessity for gas retention where it can exert its cellulating effect. For producing hollow beads of low alkali glass, the procedure has been to crush pre-formed glass of the required special composition and then to subject the resulting particles to a treatment in which gas is caused to dissolve in the glass of the particles with the aim that on subsequent spherulization of the particles, the gas will expand and cellulate the beads. Such a process is described in U.S. Pat. No. 3,365,315. Of course the production of glass for use as feedstock in such processes requires the use of a glass melting furnace operating to high temperatures at which the refractory material of the furnace is subject to considerable erosion; also the fuel costs for heating the furnace are very high.